Generic power supplies mislead on the advertised specs, are found wanting on the rails, and are not likely to deliever clean, efficient power. They entice you by adverstising, for example, 550w and charging $30. What you're seeing listed is the peak power (as opposed to sustained) under the best possible conditions. As temperatures rise, the sustained wattage decreases. When you see a generic 550w, you're seeing this peak wattage at unrealistically low temps. In other words, it's marketing.
For a modern system such as you're building, it's highly advisable not to skimp on the PSU. By going generic, you won't be getting the wattage/specs you thought you paid for. If you're dealing with a tight budget, a quality 350w is preferable to a generic 500w+.